443
Organometallics 1986, 5, 443-450
Reactions of Alkyl and Hydride Derivatives of Permethylscandocene and -2irconocene with Nitriles and Amines. Catalytic Hydrogenation of tert-Butyl Cyanide with Permethylscandocene Hydridet John E. Bercaw,” David L. Davies, and Peter T. Wolczanskit Arthur Amos Noyes Laboratory of Chemical Physics, California Institute of Technology, Pasadena, California 9 1125 Received June 17, 1985
The compounds Cp*,ScR (Cp* = q5-C5Me5;R = H, Me, p-tol) react readily with nitriles R’CN to provide azomethine complexes Cp*,ScNC(R)R’ containing a C=N double bond (u(CN) 1640-1680 cm-’1. If Cp*,ScMe is used, further reaction may occur to give products containing two nitriles. The related complex Cp*,ZrH, will insert two nitriles in a stepwise manner; however, the dimethyl analogue is unreactive toward nitriles. The compounds Cp*,ScNC(H)R may be hydrogenated under relatively mild conditions to yield the corresponding amide complexes CP*~SCNHCH~R. This transformation is reversible; addition of ethylene leads to dehydrogenation of the amide complex and production of ethane. The amide complexes may be independently synthesized by reaction of Cp*,ScR (R = H, Me) and the corresponding amine, with resultant elimination of hydrogen or methane, respectively. Cp*,ScNHCH2CMe3will catalytically hydrogenate (4 atm of H2) Me3CCN to Me,CCH,NH,; however, termination occurs by insertion of the nitrile into the scandium-amide bond to form Cp*,ScNC(CMe3)NHCH,CMe3, the NC bond of which is not hydrogenated under these conditions. Labeling experiments show that hydrogen can add reversibly across the Sc-N bond of the amide, and this step is believed to play a crucial role in the catalytic reaction.
Introduction The chemistry of scandium may be expected to be intermediate between that of early transition metals and that of yttrium and the later lanthanide elements.’ Moreover, coordinatively unsaturated organoscandium compounds appear to be voracious Lewis acids2 and in this regard resemble the organic compounds of aluminum. We have a long standing interest in the organometallic chemistry of the group IV (420)transition elements, particularly the CP*~MRR’(Cp* = T ~ - C ~ derivatives. M~~) The chemistry of the group IV metals in their higher oxidation states is dominated by their propensity to form extremely strong bonds with “hard” donor atoms such as 0, N, F, and Cl;3 a similar reactivity would certainly be expected for the even “earlier” member scandium. These strong metalheteroatom interactions are often invoked as providing the driving forces for many reactions involving substrates containing oxygen or nitrogen. Indeed, it has been generally assumed that products containing M-N or M-0 bonds are entirely too inert for such early-transition-metal complexes to function as catalysts for the reduction of substrates containing nitrogen or oxygen. This manuscript describes the reactions of compounds of the general formula Cp*,ScR and Cp*,ZrRz with N-H bonds of amines and the C=N triple bond of nitriles. The catalytic hydrogenation of tert-butyl cyanide by Cp*,ScH under moderate conditions is observed, thus belying the previous assumption, at least for nitrogen-containing substrates. A preliminary exploration of some of the reactivity associated with the Sc-N bond in these complexes, particularly that which is relevant to the hydrogenation of nitriles, is described. The general features of the chemistry of scandium appear to be intermediate between those of aluminum and early transition metals. Results and Discussion In the reaction Cp*,Sc-p-tol with p-tolunitrile, an intermediate adduct, analogous to those found for alumiFontribution No. 7205. Present address: Department of Chemistry, Cornel1 University, Ithaca, NY 14853.
*
num,” may be isolated (eq 1). The ‘H NMR spectrum for
2
1
1 displays one signal due to equivalent Cp* groups, but two resonances for the methyl substituents of the inequivalent p-tolyl groups, and the IR spectrum has a band a t 2200 cm-l characteristic of a CEN stretching mode. Compound 1 is thus formulated as the nitrile adduct, bound through the lone pair on nitrogen. In solution 1 slowly isomerizes to 2, requiring about 2 days a t 25 “C or 2 h at 80 “C. Coordination of the nitrile through the nitrogen lone pair is expected to induce polarization of the NC bond, encouraging a 1,3-alkyl shift from scandium to the carbon of the nitrile. This mechanism is entirely analogous to that proposed for the insertion of nitriles into aluminum-alkyl bonds. A study of the reactions of para-substituted phenyl cyanides with A1(NEt2)Et2suggested that r-coordination of the nitrile was not important.5 Permethylscandocene methyl reacts rapidly a t room temperature with nitriles RCN (R = Me, CMe3, CHCHJ (1) Thompson, M. E.; Bercaw, J. E. Pure A p p l . Chem. 1984, 56, 1. (2) Holton, J.; Lappert, M. F.; Ballard, D. G. H.; Pearce, R.; Atwood, J. L.; Hunter, W. E. J . Chem. SOC.,Dalton Trans. 1979, 54. (3) Wailes, P. C.; Coutts, R. S. P.; Weigold, H. ‘Organometallic Chemistry of Titanium, Zirconium, and Hafnium”; Academic Press: New York, 1974. (4) Lloyd, J. E.; Wade, K. J . Chem. SOC.1965, 2662. (5) Hirabayashi, T.; Itoh, K.; Sakai, S.; Ishii, Y. J . Organomet. Chem. 1970, 21, 273.
0276-733318612305-0443$01.50/0 0 1986 American Chemical Society
444
Organometallics, Vol. 5 , No. 3, 1986
Bercaw et al. Table I”
1, Cp*,ScC6H,Me(NCC6H4Me)
2, Cp*,ScNC(C6H,Me),
3a, Cp*,ScNC(Me),
3b, Cp*,ScNC(Me)CMe,
Me C5Me5 Me Ph Ph C5Me5 Me Ph Ph Me C5Me5
‘H NMR 1.83 (s) 2.02 (SI 2.41 (s) 6.60 (d, J 7.29 (d, J 2.03 (s) 2.17 (s) 7.04 (d, J 7.68 (d, J 1.89 (s) 1.90 (s)
CMe, Me C5Me5
1.20 (s) 1.87 (s) 1.89 (SI
3c, Cp*,ScNC(Me)CHCH2
13C NMR
= 9) = 9) = 8) = 8)
C5Me5 C(M4 CHAHB CHAHB CHCH, 4, Cp*,ScNHC(CGH40Me)CHC(C6H40Me)NH C5Me5 OMe CH NH Ph Ph
1.89 (s) 2.75 (s) 5.23 (dd, J = 10.5, 3) 5.38 (dd, J = 18, 3) 6.05 (dd, J = 18, 10.5) 1.98 (s) 3.30 (s) 5.27 (t, J = 2) 5.68 (br) 6.78 (dt, J = 8, 1.95) 7.61 (dt, J = 8.8, 1.95)
5a, Cp*,ScNC(H)CMe,
1.12 ( s ) 1.90 (s) 8.39 (s)
5b, Cp*,ScNC(H)CGH,OMe
C5Me5 OMe I-CGH4AA’BB’I CH
6, Cp*,Zr(H)NC(H)C6H4Me
7 , Cp*,Zr(I)NC(H)C,H,Me
9, Cp*,Zr{NC (H)C,H,Me},
lOc, Cp*,ScNMe, 10d, Cp*,ScNHPh
10e, Cp*,ScNHCH,CMe,
1.91 (s) 3.28 (s) 6.79 (d, J = 8) 7.40 (d, J = 8) 9.38 (s)
C5Me5 11.8 CMe, 29.8 C5Me5 117.9 CMe2 159.5 C&e5 11.8 C(Me)CMe3 24.5 CMe, 29.3 CMe3 41.2 C5Me5 117.9
C&e5
OMe CH Ph C5Me5 Ph Ph Ph (C6H40Me) C5Me5 CMe, CMe, GMe5 CH C#e5 OMe Ph
12.3 (4,J = 124) 55.2 (q, J = 143) 91.6 (dt, J = 157, 9.5) 114.3 (dd, J = 158.5, 4.4) 118.0 (s) 127.4 (dd, J = 153,b7.3) 137.8 (br)‘ 160.5 (br) 168.5 (br) 11.3 26.8 39.0 117.5 164.9 11.8 (4,J = 125) 55.2 (q, J = 143) 114.2 (d, J = 157)
C5Me5 Ph CH Ph
118.3 (s) 133.0 (dt, J = 15, 7) 154.4 (d, 161) 160.9 ( s )
1.94 (s) 2.08 (s) 4.82 (s) 7.02 (d, J = 8) 7.72 (d, J = 8) 9.23 (d, J = 1.1) N=C(H) 1.88 (s) C5Me5 2.00 (s) Me (CGH4AA’BB’J 6.99 (d, J = 8) 7.79 (d, J = 8) N=C(H) 8.54 (s) 1.82 (s) C5Me5 2.10 (s) Me l-CGH4AA’BB’J 7.18 (d, J = 8) 7.73 (d, J = 8) N=C(H) 9.69 (s) 1.90 ( s ) d C5Me5 1.95 ( ~ ) ~ , f C5Me5 C&e5 NHMe 3.52 (d, J = 7.6) NHMe 4.71 (q, J = 7.6) NHMe C5Me5 1.89 (s) C5Me5 NMe2 2.71 (s) 1.82 (s) C5Me5 4.08 (br, J = 62.5, 15N-H) NH Ph 6.11 (d, J = 7.5) 6.64 (tt, J = 7.2, 1) Ph 7.21 (t, J = 7.3) Ph CMe, 1.05 (s) 1.98 (s) C5Me5 3.47 (d, J = 8) CH2 4.75 (br) NH C5Me5 Me Zr-H (-CSH,AA’BB’)
11.3f 40.0 118.0
Organometallics, Vol. 5, No. 3, 1986 445
Reactions of Permethylscandocene and -2irconocene Table I" (Continued) 10f, Cp*,ScNHCH,C6H40Me
C5Me5 OMe NH CHZ IC,H,AA'BB'I
11, Cp*,ScNC(NHCH2CMe3)CMe3
CMe3 CMe3 C5Me5 CH2 NH
'H NMR 1.92 (s) 3.39 (s) 4.71 (br) 4.83 (d, J = 8.3) 6.94 (d, J = 8.5) 7.29 (d, J = 8.5) 0.93 (s) 1.20 (s) 1.95 (s) 3.30 (br) 4.97 (br)
I3C NMR
" All spectra are recorded in benzene-d6 a t room temperature and referenced to residual protons or carbons of solvent or to MelSi as an internal standard, unless otherwise stated. Multiplicities for I3C spectra are only reported where coupled spectra were recorded. Chemical shifts are in 6 and coupling constants in hertz. bOnly one peak of doublet observed, J calculated from the difference in frequency between line of doublet and single line in decoupled spectrum. Other phenyl signals obscured by solvent resonances. NH, protons not observed. e Spectrum obtained a t -80 "C. 'In toluene&. to form directly the azomethine insertion complexes Cp*,ScNC(Me)R (eq 2). Complexes 3a-c display a Cp*ZScMe t RCN
-
Me
Cp*zScGN=C'
(2)
R' 3a, R=Me
b , RxCMe3 C,
RzCHCH2
characteristic C=N stretching frequency in the IR spectrum at 1640-1680 cm-'. The IH and I3C NMR spectra display equivalent Cp* resonances in all cases, and for R = Me (3a),the methyl groups are also equivalent. The equivalence of Cp* ligands for 3b and 3c indicates structures for which both the Me and R groups lie in the equatorial plane. Moreover, the equivalence of the methyl groups for 3a suggests either a fluxional structure, A, or an "ylide-heteroallene" structure, B. Collier, Lappert, and Cd
'/,
.?-"C-R CP
c",
- *dc+ fast
.
+
7 //"-R
CP
I
R
A
Although Cp,Ti(Cl)NC(Me)Ph has been prepared from Cp2TiC12and LiNC(Me)Ph, the same complex was not accessible by insertion of benzonitrile into the Ti-Me bond of c ~ , T i ( M e ) C l .Moreover, ~ we have found that neither Cp*,ZrMe, nor Cp*,HfMe, will insert nitriles, possibly due to the decreased Lewis acidity of these 16e metal centers. In the reaction of Cp*,ScMe with acetonitrile to afford 3a, it is important to ensure a stoichiometry of 1:1, since excess MeCN reacts further to yield a complex which incorporates a second equivalent of MeCN. An additional competing reaction, which appears to result in loss of Cp* ligands(s),has precluded the isolation of analytical samples of this new compound.8 It has proven possible to isolate an analogous compound from the reaction of p-anisonitrile with Cp*2ScMe. The expected initial insertion product, observed (NMR) at room temperature, reacts with a second equivalent of p-anisonitrile a t 80 "C to afford 4 (eq 3). The structure of 4 is based primarily on its spectral Cp;ScCH3
30
RCN
R
/
CGSc=N=C
LCN
\
cJ(-cp*sc~y u'
Cp', Cr;
/
?-:=c
R
Y' B
McMeeking6have shown that Cp,MCl, (M = Zr, Ti) react with lithium ketimides (LiNCR,; R = Ph, p-tol, CMe,) to replace one or two chlorides. They suggested that for Cp2M(C1)NCR2(M = Zr, Ti), significant M*N=CRz *-bonding was unlikely (the 'H NMR spectrum displayed inequivalent R groups) and therefore proposed a bent MNC arrangement. However, they appear to have overlooked the fact that for these complexes both linear and bent MNC arrangements would possess inequivalent R groups, since in both cases one R would be toward the C1 and the other away. For complexes 3a-c we favor structure B above, since it not only allows the two substituents on the nitrile carbon to lie in the middle of the wedge between the Cp* ligands (i.e., in the sterically most favorable positions) but also allows for significant donation of electron density from the nitrogen lone pair into the empty bz orbital on scandium.' (6)Collier, M. R.; Lappert, M. F.; McMeeking, J. Inorg. Nucl. Chem. Lett. 1971, 7, 689.
m3
2
I
h
(3)
ZNH
R
R
4(R=?-C6H4OMel
features. Thus bands at 3325 and 3305 cm-' in its infrared spectrum indicate the presence of N-H bonds. The absence of absorptions in the ranges 2300-2100 and 1610-1690 cm-' suggests a C-N bond order of less than 2. Its lH NMR spectrum displays two equivalent OCH3 groups per [Cp*,Sc] unit, thus indicating a symmetrical arrangement of phenyl substituents. In addition there is a triplet at 6 5.27 (J = 2 Hz) with an integrated intensity of one proton and a broad signal at 6 5.68 with intensity two. The 13CNMR spectrum shows similar features and is consistent with the formulation of the complex as shown in eq 3. A possible mechanism for the formation of 4 is outlined in Scheme I. Initial coordination of the second nitrile followed by imine-enamine tautomerism (cf. keto-enol tautomerism) would generate an intermediate with a nu(7) Lauher, J. W.; Hoffman, R. J. A m . Chem. SOC.1976, 98, 1729. (8) This additional competing reaction is very likely due to the acidic nature of the methyl protons of MeCN causing protonation of one of the Cp* groups and liberation of CSMe6H.
446 Organometallics, Vol. 5, No. 3, 1986
Bercaw et al.
Scheme I
I
H
e 1
6.
R = p -C&Me
The zirconacyclodiimine 8 obtained via treatment of (Cp*zZrN2)2N2 with 4 equiv of tolunitrile reacts with dihydrogen over a period of 3 days at 70 "C to form the double nitrile insertion product 9 (eq 7 ) . An initial con-
+ i
q
~2
-
N=C
cp*zzr,
'
/R H' H ,
17)
N=C
J
8 , R=p-C6H4Me
\R 9
- 1 R
c 1 ?
4('i = p6H,0Me)
cleophilic methylene center. Attack at the electrophilic carbon of the coordinated nitrile and a second 1,3-hydrogen shift would provide the observed product. A tautomerism via just such a 1,3-hydrogen shift has been previously observed for a titanium metalla~ycle.~ Although Cp*+3cH is unstable a t room temperature in the absence of dihydrogen, it may be isolated as a T H F adduct.l Thus, reactions involving Cp*$cH were performed at -78 "C in the absence of excess hydrogen. In other cases the THF adduct was used at room temperature (see Experimental Section). Addition of nitrile to Cp*,ScH or Cp*#cH(THF) produces an instantaneous color change from off-white to bright yellow (eq 4). The NMR spectra Cp*2ScH t R C N
-
R
Cp;SckN=C'
(41
H' 5 0 , RaCMe3 b , R'p-CgH40Me C , R = p -C6H4Me
version of 8 to 6 and free p-tolunitrile is complete after 1 h at 25 "C, and the subsequent nitrile insertion to form 9 occurs over a period of days.l' Even under excess dihydrogen, there is no evidence for hydrogenation of the C=N bond to yield an amide although such a complex would be expected to be stable.12 The reaction of Cp*zScNC(H)CMe3with p-anisonitrile leads to exchange of the nitriles giving CP*~SCNC(H)C,H,OMe and Me3CCN (eq 8). Unfortunately, atU
5a H
t Me3CCN (8)
5 b mOMe
tempts to use MeCN to displace either Me3CCN or p NCC6H40Mewere not successful; decomposition occurred and free Cp*H was observed in solution.8 Thus, it is not known if the nitrile exchange reaction is driven by steric or electronic factors. Since Cp*$cH reacts very rapidly with ethylene to form p~lyethylene,'~ it is significant that Cp*,ScNC(H)R (R = CH3, CMe,, p-C6H,0Me) do not react with ethylene. We conclude therefore that nitrile exchange does not proceed via the reverse of eq 4. Rather we favor an associative mechanism such as that shown in eq 9.
r R t R'CN \H
R ?
-
!
hl
R
6 R = p - C6H4Me
absorption at 1500 cm-' and a low-field NMR resonance (6 4.82) characteristic of a Zr-H bond. A strong IR band at 1660 cm-' is indicative of the C=N bond, and the azomethine proton (6 9.23) is coupled to the Zr-H (4J = 1.1Hz). The resonances at 6 4.82 and 9.23 are absent when p-tolunitrile is treated with Cp*,ZrD2. The reaction of 6 with CH31 yields Cp*,Zr(I)NC(H)(p-C6H,Me) and 0.70 mmol of CH4/mmol of 6 (eq 6), again indicative of a Zr-H bond.1° (9)
/
Cp*ZScfN=C
of 5a,b are as expected, with the azomethine hydrogen being observed in the range 6 8.5-9.5 and the carbon at ca. 6 160. The C=N stretching frequencies are at 1640-1680 cm-l, and the mass spectra show the expected ions. In a similar manner, Cp*,ZrH, reacts at low-temperature (-78 "C) with a stoichiometric amount of p-tolunitrile to yield the monoinsertion product C P * ~ Z ~ ( H ) N = C ( H ) ( ~ - Cp*zSc=N=C' C6H4Me) (6) (eq 5). Compound 6 exhibits a broad IR
\
h
Cohen, S. A.; Bercaw, J. E. Organometallics 1985, 4, 1006.
- CP*~SCLN=C't' H
RCN
19)
R'
The amide complexes 10a-e can be prepared by reaction of Cp*@cH or Cp*,ScMe with the corresponding amine, (10) Bercaw, J. E. Adu. Chem. Ser. 1978, No. 167, 136. (11)Compound 8 undergoes exchange with free nitrile, suggesting the reductive coupling reaction which generates 8 is reversible. Thus, it is likely that 8 + H, leads initially to Cp*,ZrH, + free nitrile. Wolczanski, P. T. Ph.D. Thesis, California Institute of Technology, 1981. (12) Hillhouse, G. L.; Bercaw, J. E. J . Am. Chem. SOC.1984, 206,5472. (13) Cp*,ScH reacts very rapidly with ethylene even at -78 "C, effecting fast polymerization at 25 "C (see ref 1).
Organometallics, Vol. 5, No. 3, 1986 447
Reactions of Permethylscandocene and -2irconocene
Scheme I11
Scheme 11 N=CCM%
4
H2NCH2CMe3
Jl
IOd I
producing 1equiv of hydrogen or methane, re~pective1y.l~ It is likely that these reactions occur by interaction of the nitrogen lone pair with the scandium center followed by a four-center elimination of hydrogen or methane (eq 10 and 1l).l2The amides are formally 14e species, and some Cp*,ScR R'NH2 Cp*,ScNHR' + RH (10) loa, R' = H lob, R' = Me lOc, R' = Ph 10d, R' = CMe3 10e, R' = p-C,H,OMe R = H, CH3 CP*~SCCH +~"Mez Cp*,ScNMez + CH4 (11) 1oc N Sc *-donation is thus anticipated. However, to allow donation from the nitrogen lone pair, the substituents on nitrogen must be directed toward the Cp* ligands, Le., in the sterically least favorable conformation. It has been shown12 that for the related Cp*,HfH(NR2) system, the preferred conformation for R = Me has the methyl groups lying in the equatorial plane between the Cp* groups, Le., an ortliogonal orientation for *-bonding. However, models for the parent complex CP*~H~'H(NH,) suggested no prohibitive steric interactions between the NH2 hydrogens and the Cp* groups. Accordingly, the low-temperature lH NMR spectra (500 MHz, -90 "C) of C P * ~ H ~ H ( N H dis~) plays equivalent amide proton resonances consistent with the H f k N H 2 conformation. The low-temperature 'H NMR spectrum of Cp*,ScNHMe (400 MHz, -80 "C) shows equivalent Cp* moieties, indicating either that the preferred conformer is that with no nitrogen *-donation or that rotation about the Sc-N bond is still rapid a t this temperature. The amide complexes CP*~SCNHCH~R (10e,f) can also be prepared by hydrogenation of the complexes 5a,b. Interestingly, this hydrogenation is reversible, since reaction of the amide with ethylene a t 80 "C gives the azomethine and ethane (eq 12).
+
-
-
-
The mechanism for hydrogenation of 5a,b to 10e,f has been probed by treating Cp*,Sc15NHPh with D,. Rapid exchange to afford Cp*,Sc15NDPh and HD is observed by lH NMR. No incorporation of deuterium into the Cp* positions is observed, so that mechanisms involving ligand C-H bonds (i.e., via "tuck-in" intermediates) are eliminated. Thus, we favor a four-center mechanism like that postulated to explain C-H activation by this system' (Scheme 11). Consistent with the proposed equilibrium between C P * ~ S C N H PH2, ~ , and [Cp*2Sc(H)(NH2Ph)]in Scheme I1 is the observation that ethylene is rapidly hydrogenated to ethane by 10d, whereas in the absence of H2no reaction between 10d and CZH4 is observed (eq 13).15 C2H4
+ "2
Cp*&NHPh
' CZH6
The observation that 5a,b can be hydrogenated under relatively mild conditions suggests that catalytic hydrogenation of nitriles may be possible in this system. The reaction of Cp*2ScNHCH,CMe3(5a) with Me3CCN under dihydrogen (4 atm) proceeds at 80 "C for a few turnovers to liberate NH2CH2CMe3.However, there is a competing, irreversible reaction whch removes the catalytic species, involving insertion of nitrile into the Sc-N bond of the amide to form 11, analogous to similar reactions previously observed5 for aluminum amides (eq 14). Cp*zScNHCH2CMe3 t Me3CCN 10e
-
Cp;Sc&N=C
/
NHCH2CMe3 (14)
h e 3 11
A likely mechanism for the catalytic cycle, based almost entirely on precedented steps, is shown in Scheme 111. It is worth noting once again that the key step in this mechanism is addition of dihydrogen across a Sc-N bond. Conclusions
5a, R=CMe3 b, R = P - C ~ H ~ O M ~
10e. R=CMe3 f,
R'P-C~H~OM~
(14) An exception to eq 10 and 11 is the reaction of Cp*,ScCH, with p-NH,CH,C6H40Me for which the product isolated is the amine adduct Cp*~cNHCH,C,H,0Me(NHzCHzC~40Me). The adduct appears to be fluxional ('H NMR), hydrogen transfer from amine to amide interconverting the two groups. At low temperature this process can be frozen out (-80 OC,400 MHz): C5Me5(6 2.00, (s)), OMe (6 3.25 (s)), OMe (6 3.28 (s)),CHZ (6 3.50 (b s)), CH2 (6 4.69 (b s)), Ph (6 6.74), (d, J = 8.2 Hz),6.93 (d, J = 8.6 Hz), 6.96 (d, J = 8.6 Hz), 7.50 (d, J = 8.3 Hz)). Reaction with Cp*,ScMe affords CHI and Cp*2ScNHCHzC6H,0Me;however, isolation of the amine adduct of the amide has not been possible. Evidence for other amine-amide adducts has also been obtained from NMR experiments, but their stability is even lower than that for the above compound.
The reactions of permethylscandocene and -zirconocene hydrides with nitriles proceed rapidly to provide the azomethine insertion products. Moreover, permethylscandocene alkyl and aryl complexes react in a similar manner, and in one case, an intermediate nitrile adduct of an aryl complex has been directly observed. These reactions are very similar to those of aluminum hydrides and alkyls with n i t r i l e ~ ; ~initial f ~ J ~nitrile adducts of alu(15) An alternative mechanism involving free Cp*,ScH cannot be ruled out; however, it is expected to react much faster and yield polyethylene under these conditions.
448
Organometallics, Vol. 5, No. 3, 1986
minum alkyls have been isolated in several cases which, when heated, give the carbalumination products. As far as we are aware, there are, as yet, no examples of transfer of an alkyl or aryl group from a transition metal to a nitrile. There have, however, been examples of insertion of a nitrile into an actinidecarbon bond; tert-butyl cyanide was found to insert into the M-C bond of the metallacycles
,
.
[(Me3Si),NI2MCH2Si(Me2)NNSiMe3 (M = Th or U) to give six-membered meta1lacycles.l' There are many examples of insertion of a nitrile into a transition-metal hydride bond, although more commonly for cluster compounds.18 The driving force for the reactions of permethylscandocene and -zirconocene derivatives is presumably the high Lewis acidity of these early transitionmetal complexes, which polarizes the N d ! bond and thus encourages alkyl transfer. The reaction of nitriles with Cp*2ScMe is faster than with aluminum alkyls. Accordingly, deprotonation of the a-carbon does not compete with insertion of the nitrile; however, the higher reactivity of the scandium products may result in further reactions with nitriles bearing hydrogens on the a-carbon (e.g., eq 3). The nucleophilicity of scandium hydrides or alkyls has been used to prepare a variety of amide complexes from the corresponding amines. This same type of reactivity has been observed previously for permethylzirconocene and permethylhafnocene hydrides and may be generally applicable for the synthesis of early metal amide complexes. The catalytic hydrogenation of nitriles by organometallic complexes is well pre~edented,'~ and presumably nitrile insertion into metal-hydrogen bonds also occurs in these systems. However, stepwise reduction has only been observed in noncatalytic systems where liberation of the amine leads to decomposition of the complex.18b,c Our observation of stepwise hydrogenation of nitriles is believed to be the first in a catalytic system. It is surprising that such catalysis occurs with an early transition metal such as scandium for which the Sc-N bond had been expected to be very resistant to hydrogenation. It is particularly significant that a Sc-N bond undergoes hydrogenolysis under relatively mild conditions. The implication is that under suitable conditions early transition elements may serve as viable homogeneous catalysts, even for substrates containing heteroatoms such as nitrogen. Experimental Section General Considerations. All manipulations were performed by using glovebox or high vacuum techniques, as previously described. IR spectra were recorded on a Beckman 4240 spectrophotometer as Nujol mulls and are reported in inverse centimeters. Proton, 2H, and I3C NMR spectra were recorded by using a Varian
(16) (a) Reinheckel, H.; Jahnke, D. Chem. Ber. 1964, 97, 2661. (b) Pasynkiewicz, S.; Maciaszek, S. J. Organomet. Chem. 1968,15, 301. (c) Miller, A. E. G.; Bliss. J. W.; Schwartzman, L. H. J . Org. Chem. 1959,24, 627. (17) Simpson, S. J.; Andersen, R. A. J . Am. Chem. Soc. 1981, 103, 4063. (18) (a) Mays, M. J.; Prest, D. W.; Raithby, P. R. J. Chem. Soc., Chem. Commun. 1980,171. (b) Andrews, M. A,; Kaesz, H. D. J. Am. Chem. SOC. 1979,101,7238. (c) Bernhardt, W.; Vahrenkamp, H. Angew. Chem.,Int. Ed. Engl. 1984, 23, 381. (d) Evans, W. J.; Meadows, J. H.; Hunter, W. E.; Atwood, J. L. J. Am. Chem. Soc. 1984, 106, 1291. (19) (a) Teshikatsu, Y.; Okano, T.; Otsuka, S. J. J . Chem. Soc., Chem. Commun. 1979, 870. (b) Grey, A. R.; Pez, G. P.; Wallo, A. J . Am. Chem. SOC.1981, 103, 7536. (20) In this paper the periodic group notation in parentheses is in accord with recent actions by IUPAC and ACS nomenclature committees. A and B notation is eliminated because of wide confusion. Groups IA and IIA become groups 1 and 2. The d-transition elements comprise groups 3 through 12, and the p-block elements comprise groups 13 through 18. (Note that the former Roman numeral designation is preserved in the last digit of the new numbering: e.g., 111 3 and 13.)
-
Bercaw et al. E M 390 or JEOL GX 400 spectrometer. Spectra were recorded in benzene-d6 or toluene-d, solution and are referenced to Me4Si or residual protons or carbons of solvent. Elemental analyses and mass spectra were recorded by Larry A. Henling at the California Institute of Technology. Dimethylamine (Eastman Kodak), neopentylamine, aniline, and p-methoxybenzylamine (Aldrich) were stored over 4-8, molecular sieves. Anhydrous ammonia and methylamine (Matheson) were freeze-pump-thawed twice before use. I5N aniline (MSD) was used as supplied. Many of the syntheses reported proceed quantitatively (NMR),but the isolated yields are often low because the compounds are extremely soluble in hydrocarbon solvents. Cp*,Sc(p-tol)p-NCC6H4Me (I). Cp*,Sc(p-tol) (400 mg, 0.98 mmol) and p-tolunitrile (150 mg, 1.28 mmol) were stirred in 10 mL of petroleum ether, giving a yellow precipitate. Recrystallization from petroleum ether gave 1: 407 mg (79%); IR 3020 (s), 2708 (m), 2245 (vs), 1602 (s), 1500 (m), 1405 (m), 1222 (m), 1205 (w), 1172 (s), 1120 (m), 1052 (sh), 1030 (s), 1014 (s), 815 (s), 808 (sh), 778 (s), 712 (w), 582 (w), 545 (m), 480 (m) cm-'. Anal. Calcd for C35H3,NSc: C, 80.27; H, 8.47; N, 2.67. Found: C, 79.9; H , 8.55; N, 2.83. Cp*&kNC(p-tol), (2). Cp*,Sc(p-tol) (263 mg, 0.65 mmol) and p-tolunitrile (104 mg, 0.89 mmol) were heated at 80 "C in toluene (5 mL) for 2.5 h. Recrystallization from petroleum ether gave 2 as an orange-yellow solid: yield 65%; IR 2720 (w), 1638 (vs), 1605 (s), 1567 (w), 1505 (m), 1302 (w), 1281 (w), 1240 (m), 1208 (w), 1175 (s), 1103 (w), 1018 (m), 907 (m), 827 (s), 815 (s), 778 (w), 732 (m), 565 (w), 544 (w), 463 (w), 425 (m) cm-'. Anal. Calcd for C3SH,,NSc: C, 80.27; H, 8.47; N, 2.67. Found: C, 80.00; H, 8.25; N, 3.66. Cp*,ScNC(Me)CHCH, (312). Acrylonitrile (77 wL, 1.17 mmol) was added to a stirred solution of Cp*,ScMe (387 mg, 1.17 mmol) in petroleum ether (15 mL). After being stirred for 0.5 h, the solution was concentrated to ca. 2 mL, cooled to -78 "C, and filtered to give a yellow solid, 3c: yield 200 mg (45%); IR 3285 (2), 3085 (m), 2722 (m), 1820 (w), 1645 (vs), 1610 (vs), 1345 (SI, 1275 (m), 1218 (s),1162 (w), 1145 (w), 1067 (m), 1020 (s), 992 (s), 944 (m), 908 (s), 795 (m), 740 (w), 705 (m), 675 (sh), 658 (m) cm-'. Anal. Calcd for C24H36NS~: C, 75.16; H, 9.46; N, 3.65. Found: C, 74.46; H, 9.43; N, 3.31. Similar procedures were used for the preparation of Cp*,ScNC(Me)CMe3 (3b) and Cp*,ScNC(Me), (3a). Compound 3b: yield 32%; IR 2720 (w), 1672 (vs), 1630 (m), 1433 (s), 1398 (m), 1358 (m), 1343 (m), 1242 (m), 1210 (w), 1195 (w), 1150 (m), 1102 (s), 1058 (m), 1015 (m), 947 (m), 868 (w), 832 (w), 798 (w), 730 (w), 700 (w), 565 (w), 504 (w), 423 (s) cm-'. Anal. Calcd for CBH4,NSc: C, 75.51; H, 10.24; N, 3.39. Found: C, 74.33: H, 10.13; N, 3.08. Compound 3a: yield 56%; IR 1790 (w), 1710 (s), 1685 (vs), 1655 (sh), 1648 (sh), 1482 (m), 1340 (s), 1178 (s), 1058 (w), 1020 (s), 892 (w), 798 (w), 530 (m), 478 (s), 415 (s) cm-'. Anal. Calcd for CZ3H,,NSc: C, 74.36; H , 9.77; N, 3.77. Found: C, 73.39; H, 9.54; N, 3.49.
Cp*2ScN(H)C(C6H40Me)C(H)C(C6H40Me)N(H) (4). A mixture of Cp*,ScMe (400 mg, 1.21 mmol) and p-NCC6H40Me (330 mg, 2.48 mmol) was heated a t 80 "C for 3 days in toluene (10 mL). The toluene was removed and the residue extracted with petroleum ether (10 mL). Concentration to ca. 2 mL, cooling to -78 "C, and filtration gave a lemon yellow solid: yield 53%; IR 3325 (w), 3305 (w), 1605 (s), 1575 (m), 1545 (s), 1525 (s), 1498 (vs), 1412 (s), 1398 (s),1290 (s),1250 (vs), 1215 (m), 1170 (s). 1102 (w), 1025 (s), 915 (w), 835 (s), 812 (w), 765 (s), 732 (m), 718 (w), 680 (m) cm-'. Anal. Calcd for C37H47N202S~: C, 74.47; H, 7.94: N, 4.70. Found: C, 71.49; H, 7.53; N, 4.56. Cp*,ScNC(H)CMe3 (5a). A toluene (10 mL) solution of Cp*,ScMe (404 mg, 1.22 mmol) in a thick walled glass reaction vessel with Teflon needle valve was cooled to -196 "C, dihydrogen (700 torr) was admitted, and the valve was closed. The solution was warmed to room temperature with stirring and was stirred for a further 15 mins. The solution was cooled to -78 "C, and the valve was opened to vacuum to remove dihydrogen. tert-Butyl cyanide (140 fiL, 1.27 mmol) was added, causing an immediate color change from cream to yellow. The mixture was stirred for 30 min, the toluene was removed, and petroluem ether (5 mL) was added. The solution was transferred to a frit, concentrated
Reactions of Permethylscandocene a n d -2irconocene
to ca. 2 mL, cooled to -78 "C, and filtered to isolate a pale yellow solid: yield 38%; IR 3360 (w), 2710 (s), 2625 (s), 1748 (m), 1675
(s), 1625 (m), 1240 (m), 1202 (SI, 1148 (w), 1082 (m), 1055 (sh), 1024 (s), 885 (sh), 870 (m), 795 (w), 748 (m), 692 (m), 645 (m) cm-'. Anal. Calcd for C,,H,,NSc: C, 75.15; H, 10.09; N, 3.51. Found: C, 75.00; H , 10.23; N, 3.54. Cp*,ScNC(H)C6H40Me (5b). Cp*,ScMe (1.13 g, 3.42 mmol) was stirred in T H F (10 mL) under dihydrogen (1 atm) for 17 h to form Cp*,Sc(H)THF. p-NCC6H40Me(500 mg, 3.76 mmol) in petroleum ether (10 mL) was added, and the mixture was stirred for 0.5 h. The solution was evaporated to dryness, and petroleum ether (5 mL) was added. Concentration to ca. 2 mL and cooling to -78 "C allowed isolation of a yellow solid: yield 27%; IR 1648 (vs), 1595 (vs), 1572 (s), 1495 (s), 1350 (s), 1302 (s), 1242 (vs), 1152 (s),1092 (w), 1025 (s), 828 (s), 800 (m), 640 (m) cm-'. Anal. Calcd for C,8H38NOSc: C, 74.80; H, 8.52; N, 3.12. Found: C, 71.71; H, 8.21; N, 2.93. Cp*,Zr(H)NC(H)C6H,Me (6). Cp*,ZrH2 (200 mg, 0.550 mmol) and p-tolunitrile (64 mg, 0.550 mmol) were placed in a flask, and toluene (15 mL) was distilled in at -78 "C. After the solution was warmed to 25 "C and stirred for 1h, the solvent was removed. Recrystallization from petroleum ether yielded bright yellow 6: yield 57%; v(C=N) 1660, v(Zr-H) 1500 cm-'. Anal. Calcd for C28H39NZr:C, 69.94; H, 8.18; N, 2.91. Found: C, 68.77; H, 8.16; N, 1.93. Cp*,Zr(I)NC(H)C6H,Me (7). Cp*,Zr(H)NC(H)C6H4Me (200 mg, 0.416 mmol) was placed in a flask, and toluene (15 mL) was distilled in along with about 4 mmol of MeI. After the solution was warmed to 25 "C, the gas was collected (0.291 mmol, 0.70 mmol/mmol of 6). After passage over hot CuO, the volume of gas was unchanged (IR identified CH4). The solution was filtered, and the toluene was then removed. The resulting yellow powder was slurried in petroleum ether and then collected (160 mg, 63% yield): v(C=N) 1650 cm-'. Anal. Calcd for CBHSNIZr: C, 55.43; H , 6.31; N, 2.31. Found: C, 55.49; H, 6.34; N, 2.23. Cp*,Zr(NC(H)C6H4MeJz(9). Compound 8 was prepared by treatment of (Cp*,ZrNJ2N2 (500 mg, 0.62 mmol) with 290 mg (2.48 mmol) of p-tolunitrile in 50 mL of toluene, -78 25 "C. After 4 h a t 25 "C, toluene was removed and 8 crystallized from petroleum ether (40 mL). Orange crystalline 8 (650 mg, 82%) was obtained. Anal. Calcd for C,,H,N,Zr: C, 72.54; H, 7.46; N, 4.70. Found: C, 72.36; H, 7.57; N, 4.49. A glass bomb reactor (65 mL) containing 173 mg of 8 (0.291 mmol) and 10 mL of toluene was pressurized with 1.254 mmol of H, and heated for 5 days a t 70 "C to ensure complete reaction. After this time the unreacted H2 was Toepler pumped (0.979 mmol); thus, 0.275 mmol of H, were absorbed (0.945 mmol/mmol of 8). The brown solution was then transferred to a flask, and pure 9 was crystallized from toluene/petroleum ether. The brown-orange crystals were collected on a frit (156 mg, 90% yield): v(C=N) 1670 cm-'. N i t r i l e E x c h a n g e . An N M R tube was loaded with Cp*,ScNC(H)CMe, (25 mg, 0.05 mmol), p-NCC6H40Me (9 mg, 0.07 mmol), and 0.3 mL of benzene-d,. The NMR spectrum showed that complete exchange had occurred; addition of 10 equiv of Me,CCN did not produce a detectable amount of Cp*,ScNC(H)CMe,. Hydrogenation of Cp*,ScNC(H)R (5a,b). An NMR tube sealed to a ground-glass joint was loaded with Cp*,ScNC(H)CMe3 (20 mg, 0.05 mmol) and 0.3 mL of benzene-d6. The tube was cooled to 77 K, 700 torr of H 2 introduced, and the tube sealed with a torch. The formation of Cp*,ScNHCHzCMe3 was complete after 24 h a t room temperature. Similarly, hydrogenation of Cp*,ScNC(H)C6H40Me t o Cp*zScNHCH,C6H40Me was complete within 6 h a t room temperature. Dehydrogenation of Cp*2ScNHCH2CMe3(10e). An NMR tube sealed to a ground-glass joint was charge.d with Cp*,ScNHCH,CMe, (20 mg, 0.05 mmol) and 0.3 mL of benzene-d,. The tube was cooled to 77 K, ethylene (58.6 torr in 15.8 ml, 0.05 mmol) condensed in, and the tube sealed with a torch. The tube was heated a t 80 "C for 17 h after which time the ethylene had been converted to ethane and complex 10e has been completely converted to Cp*,ScNC(H)CMe, (5a); no intermediates were observed. Catalytic Hydrogenation of tert -Butyl Cyanide. An NMR tube sealed t o a ground-glass joint was charged with
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Organometallics, Vol. 5 , No. 3, 1986 449 Cp*,ScNHCHzCMe3 (20 mg, 0.05 mmol) and 0.4 mL of benzene& The tube was cooled to 77 K, Me3CCN (28 torr, 108 ml, 0.16 mmol) was condensed in, 700 torr of H2was introduced, and the tube was sealed with a torch. Complex 5a was observed as soon as the spectrum was recorded. The reaction proceeds slowly a t 25 "C, and complex 11 could be observed. After 17 h the tube was heated to 80 "C, and after a further 24 h of heating there was virtually no tert-butyl cyanide left, 2 equiv of NH2CH2CMe3had been formed, and the major scandium-containing species (>70%) was complex 11. Cp*,ScN=C(NHCHCMe3)CMe3 (11). tert-Butyl cyanide (75 pL, 0.68 mmol) was added t o a stirred solution of Cp*,ScNHCH,CMe, (266 mg, 0.66 mmol) in petroleum ether (15 mL). After being stirred for 17 h, the solution was concentrated to ca. 2 mL, cooled to -78 "C, and filtered, to provide 11 as a white solid in 31% yield IR 3466 (m), 3398 (w), 2718 (w), 1627 (s), 1345 (vs), 1247 (m), 1210 (vs), 1185 (s), 1087 (w), 1063 (w), 1017 (m), 922 (w), 892 (w), 798 (w), 742 (m), 717 (w) cm-'. Anal. Calcd for C&51NZS~: C, 74.34; H, 10.67; N, 5.78. Found: C, 68.82; H, 9.89; N, 5.29. Cp*,ScNH, (loa). Anhydrous ammonia (450 torr in 108 mL, 2.6 mmol) was condensed into a solution of Cp*,ScMe (848 mg, 2.57 mmol) in petroleum ether (10 mL) a t -196 "C. The solution warmed to room temperature with stirring and was stirred for a further 0.5 h. Recrystallization from petroleum ether gave an off-white solid: yield 26%; IR 3382 (w), 3356 (sh), 3285 (w), 1592 (m), 1525 (vs), 1490 (m), 1220 (m), 1170 (sh), 1155 (s), 1058 (w), 1022 (s), 935 (w), 800 (w), 648 (m) cm-'. Anal. Calcd for C2,H,,NSc: C, 72.47; H, 9.73; N, 4.23. Found: C, 70.03; H , 9.67; N, 5.19. Similar procedures were used for the isolation of Cp*zScNHMe (lob) and Cp*2ScNMe2 (1Oc). Compound Cp*,ScNHMe: yield 37%; IR 3360 (w), 3270 (w), 2770 (s), 2730 (sh), 1585 (w), 1145 (w), 1085 (s), 1042 (s), 1018 (s), 992 (sh), 970 (m), 800 (w), 655 (m) cm-'. Anal. Calcd for C21H34NS~: C, 73.01; H, 9.92; N, 4.05. Found: C, 72.00; H , 9.55; N, 3.92. Compound Cp,ScNMe,: yield 38%; IR 2752 (s), 2720 (sh), 1234 (s), 1157 (s), 1116 (w), 1017 (m), 922 (s), 795 (w), 712 (w), 650 (w), 490 (w), 368 (m) cm-'. Anal. Calcd for CzzH36NSc: C, 73.50; H, 10.09; N, 3.90. Found: C, 73.33; H , 10.07; N, 3.85. With the less volatile amines NHzR (R = P h , CH,CMe,, p CH2C6H,0Me) 1 equiv of amine was syringed into a stirred solution of Cp*,ScMe. Recrystallization from petroleum ether provided the corresponding amides Cp*,ScNHPh (10d) and Cp*,ScNHCHCMe, (1Oe). However for p-NH2CH2C6H40Me the complex isolated is the adduct Cp*,Sc(NHCH2C6H40Me)NH,CH,C6H40Me (lz).', Compound Cp*,ScNHPh: yield 47%; IR 3700 (w), 2720 (w), 1578 (s), 1560 (m), 1342 (m), 1277 (vs), 1192 (w), 1168 (m), 1143 (w), 1058 (w), 1014 (m), 882 (m), 830 (m), 736 (s), 681 (m) cm-'. Anal. Calcd for CZ6H3,NSc: C, 76.62; H, 8.90; N, 3.44. Found: C, 75.59; H, 8.72; N, 3.53. Compound Cp*,ScNHCH,CMe,: yield 33%; IR 3370 (w), 3342 (w), 2770 (m), 2720 (w), 1388 (s), 1358 (m), 1318 (w), 1275 (w), 1235 (w), 1207 (w), 1152 (w), 1095 (s), 1018 (m), 1000 (m), 925 (w), 798 (w), 740 (w), 658 (m), 610 (m) cm-'. Anal. Calcd for C,,H,,NSc: C, 74.78; H, 10.54; N, 3.49. Found: C, 74.00; H , 10.44; N, 3.42. Compound Cp*,Sc(NHR)NH2R (R = CH2C6H40Me):yield 14%; IR 3705 (w), 3360 (m), 3300 (w), 1608 (s), 1578 (s), 1505 (vs), 1335 (s), 1300 (s), 1238 (vs), 1202 (w), 1172 (s), 1130 (m), 1105 (s), 1072 (s), 1032 (s), 960 (s), 838 (m), 825 (m), 802 (s), 745 (s), 700 (w) cm-'. Anal. Calcd for C3,H,2N,0,Sc: C, 71.74; H, 8.90; N, 4.76. Found: C, 71.99; H, 8.82; N, 4.22. Reaction of Cp*,Sc(NHR)NH2R (R = CH,C6H40Me) with Cp*,ScMe.14 An N M R t u b e was charged with Cp*,ScNHCHzC6H40Me(NH2C",0Me) (20 mg, 0.034 mmol), Cp*,ScMe (18 mg, 0.054 mmol), and 0.4 mL of benzene-ds. An immediate reaction occurred with liberation of a gas; observation of the NMR spectrum showed the presence of Cp*,ScNHCH2C6H4OMe (lOf), methane, and unreacted Cp*,ScMe.
Acknowledgment. This work w a s s u p p o r t e d by the N a t i o n a l Science F o u n d a t i o n ( G r a n t No. CHE-8024869).
450
Organometallics 1986, 5, 450-455
Use was made of the Southern California Regional NMR Facility (NSF Grant No. CHE-7916324). D.L.D. a&nowldeges support through a NATO Postdoctoral Fellowship administered through the Science and Engineering Research Council (U.K.). Registry No. 1,99706-97-5;2,99706-98-6;3a, 99706-99-7; 3b, 99707-00-3;3c, 99707-01-4; 4, 99707-02-5; 5a, 99707-03-6;5b, 99707-04-7; 6, 99707-05-8; 7, 99707-06-9; 8, 99707-07-0; 9,
99707-16-1; loa,99707-081; lob,99707-09-2; ~ O C 99707-10-5; , LOd, 99707-11-6; 10e, 99707-12-7; 10f, 99707-17-2; 11, 99707-13-8;
C P * ~ S ~ ( ” C H Z C , H * O M ~ ) N H Z C H , C ~ H , O M99725-86-7; ~,
CP*~Scb-tol),99707-14-9; CP*zScMe, 99707-15-0; CP*ZZ~H~, 61396-34-7;(Cp*zZrNz)zNz, 54387-50-7;p-NCCsH40Me,874-90-8; NH2CHzCMe3,5813-64-9;p-tolunitrile, 104-85-8;acrylonitrile, 107-13-1;tert-butyl cyanide, 630-18-2;acetonitrile, 75-05-8;ammonia, 7664-41-7;methylamine, 74-89-5;dimethylamine,124-40-3; aniline, 62-53-3;4-methoxybenzenemethamine, 2393-23-9.
Photophysics and Photochemistry of a Series of M(CO),L Complexes Where M = Cr or Mo and L = Pyridine or a Substituted Pyridine Richard M. Kolodziej and Alistair J. Lees* Department of Chemistry, State University of New York at Binghamton, Binghamton, New York 13901 Received July 25, 1985
Electronic absorption data are reported for a series of M(CO)5Lcomplexes, where M = Cr and Mo and L = pyridine or a substituted pyridine. Low-lying ligand field (LF) and metal to ligand charge-transfer (MLCT) transitions are observed in the electronic absorption spectra. As the pyridine substituent becomes more electron withdrawing, the MLCT feature shifts to lower energy. For L = pyridine, 4-methylpyridine, 3,5-dichloropyridine,4-phenylpyridine, 2-cyanopyridine,and 3-cyanopyridine the LF transition is the lowest lying excited state. For L = 4-cyanopyridine,4-benzoylpyridine,4-acetylpyridine, and 4-formylpyridine the MLCT transition is at lowest energy. Luminescence has been detected from room-temperature solutions of those complexes with a lowest lying MLCT transition and thus assigned to originate from this state. Upon excitation in the visible region these complexes undergo photosubstitution of the pyridine ligand. The photosubstitution quantum yields are most sensitive to the nature of the lowest energy excited state, being considerably greater when this state is of LF character as opposed to MLCT character. The quantum efficiencies were determined not to be thermally activated following MLCT excitation, suggesting that another state, possibly a LF triplet, is close in energy to the low-lying MLCT transition and is responsible for the photochemistry. Excited-state schemes based on the experimental data are reported.
Introduction Unlike classical coordination compounds, the photochemical properties of organometallic species are only poorly understood. Yet a further knowledge of these photoprocesses may be expected to prove extremely Valuable toward successful design of synthetically useful and catalytically active transformations. It has been a focus of our research to study excited-state deactivation pathways in transition-metal organometallic complexes and, specifically, to learn of the interrelationship of their photophysical and photochemical behavior. Luminescence characteristics of transition-metal organometallic complexes have not been explored in detail. Most investigations of the emission behavior of organometallic complexes have been carried out a t low temperature in a rigid environment, such as a glass or a matrix, where nonradiative deactivation pathways are substantially reduced.’ When examples of emission from metal complexes in fluid solution are discovered, valuable information on excited state processes can be obtained.2~~In particular, metal carbonyl complexes have not been thought to luminesce in fluid solution due to the relatively high photoreactivity and efficient nonradiative deactivation routes (1) Calvert, J. G.; Pitts, J. N. “Photochemistry”;Wiley-Interscience: New York, 1966; p 240. (2) Fleischauer, P. D.; Fleischauer, P. Chem. Reu. 1970, 70, 199. (3) Porter, G. B. “Concepts of Inorganic Photochemistry”;Adamson, A. W., Fleischauer, P. D., Eds.; Wiley-Interscience: New York, 1975; p 37.
0276-7333/86/2305-0450$01.50/0
of their excited states. Only a few complexes are known to emit in room-temperature conditions, The lack of available data has, up to the present time, inhibited models for the excited states of transition-metal carbonyls and their derivatives. Several C1Re(C0)3Lcomplexes, where L = 1,lO-phenanthroline and related ligands, have been reported to luminesce in fluid solution, and the emission attributed to a low-energy MLCT state with considerable triplet ~ h a r a c t e r .Room-temperature ~ luminescence from a MLCT triplet state has also been observed from a series of XRe(C0)3L, complexes, where X = C1, Br, I, and L = 4,4’-bipyridine, 4-phenylpyridine, and 3-ben~oylpyridine.~ Upon cooling this system was concluded to undergo substantial reordering of low-lying states; the emission at 77 K attributed to an intraligand (a-a*)triplet state. Recently emission in fluid solution has been determined from a series of W(CO)6L complexes, where L = a pyridine derivative! Noticeably all these examples are of third-row transition-metal systems in which spin-orbit coupling parameters can be expected to be higha7 We have, (4) (a) Wrighton, M.; Morse, D. L. J. Am. Chem. SOC.1974,96, 998. (b) Giordano, P. J.; Fredericks, S. M.; Wrighton, M. S.; Morse, D. L. J . Am. Chem. SOC. 1978, 100, 2257. (c) Fredericks, S. M.; Luong, J. C.; Wrighton, M. S. J . Am. Chem. SOC.1979, 101, 7415. (d) Luong, J. C.; Faltynek, R. A.; Wrighton, M. S. J. Am. Chem. SOC. 1979, 101, 1597. (5) Giordano, P. J.; Wrighton, M. S. J. Am. Chem. SOC. 1979, 101, 2888. (6) (a) Lees, A. J.; Adamson, A. W. J.Am. Chem. SOC.1980,102, 6874. (b) Lees, A. J.; Adamson, A. W. J. Am. Chem. SOC.1982, 104, 3804. (7) Moore, C. E. “National Standards Reference Data Service”;National Bureau of Standards: Washington, D.C., 1971; Vol. 3.
0 1986 American Chemical Society
